The invention relates to a thin film transistor liquid crystal display (TFT-LCD) array substrate and a manufacturing method thereof.
A thin film transistor liquid crystal display (TFT-LCD) has the advantage of small volume, low energy consumption, low radiation, and etc., and thus prevails in the flat panel display market.
A TFT-LCD typically constitutes of an array substrate and a color filter substrate that face each other. The array substrate is provided with thin film transistors and pixel electrodes, arranged in matrix, each pixel electrode being controlled by a thin film transistor. The pixel electrode is charged when the thin film transistor is on. After charging, the voltage of the pixel electrode remains unchanged until recharging at the time of next scanning. Generally speaking, the capacitance of liquid crystal is small; and thus mere capacitance of the liquid crystal is not able to sustain the voltage of the pixel electrode. For this reason, it is worth providing a storage capacitor to sustain the voltage of the pixel electrode. In principal, a storage capacitor is categorized as a storage capacitor on a gate line (Cs on Gate), a storage capacitor on a common electrode line (Cs on Common), or a combination structure thereof. The combination structure refers to a structure where a portion of the storage capacitors is formed on a gate line and another portion is formed on a common electrode line.
When a TFT-LCD operates, a kickback (jump) voltage ΔVp is generated at the moment the charging of the pixel electrode is finished, due to the parasitic capacitances between the source and the gate electrode and between the drain and the gate electrode. The kickback voltage ΔVp is expressed as follows:
            Δ      ⁢                          ⁢              V        p              =                  (                              V            gh                    -                      V            gl                          )            ⁢                        C          gs                                      C            gs                    +                      C            lc                    +                      C            s                                ,where Vgh stands for a turn-on voltage of the gate electrode, Vgl is a turn-off voltage of the gate electrode, Clc signifies the liquid crystal capacitance, Cgs is the parasitic capacitance, and Cs stands for the storage capacitance. Research has shown that the polarity of the pixel electrode varies due to the kickback voltage ΔVp, and thus the voltage difference between the positive and the negative polarity varies, which causes flickering of the display and thus severe deterioration of the display quality. Thereupon, it is necessary to reduce the kickback voltage ΔVp as much as possible, for the sake of designing. Although the kickback voltage ΔVp can be reduced by reducing the parasitic capacitance Cgs, it is unfeasible to totally eliminate the parasitic capacitance, due to limit of the TFT-LCD manufacturing process. The prior art reduces the kickback voltage ΔVp through increasing the storage capacitance Cs, which, however, has the following problems:
(1) Regarding the storage capacitor on the gate line, the kickback voltage ΔVp can not be effectively reduced, because the capacitance of the storage capacitor is small;
(2) Regarding the storage capacitor on the common electrode line, the kickback voltage ΔVp can be, indeed, effectively reduced, thanks to the large capacitance of the storage capacitor, nevertheless, typically made of gate metal thin film materials, the common electrode line blocks a portion of the pixel region, and thus decreases the aperture ratio and the display luminance;
(3) Regarding the combination structure, though alleviating the above two problems, the combination structure renders the TFT-LCD structure complicated, which adds complexity to the manufacturing process and adversely influences the product quality.